LVDS (Low Voltage Differential Signaling), specified in 1995 as a standard for a short-range digital communications, is widely applied to interfaces which transmit differential signals. Differential signal transmission by LVDS is characterized by low power consumption, excellent noise immunity and little electromagnetic radiation.
The maximum transmission rate specified for LVDS was conventionally 3.125 Gbps in view of performance of differential transmission cables and semiconductor elements. Now, improvement in performance of semiconductor elements due to miniaturization, etc., allows transmission signal to be output at not less than 28 Gbps and a differential transmission cable with metal capable of differential signal transmission of not less than 28 Gbps has been put to practical use. The transmission range of this differential transmission cable is limited to a very short range.
Meanwhile, since differential signals transmitted through differential transmission cables are also required to exhibit a quality transmission waveform, signal conditioning techniques for differential transmission has been also significantly improved. For example, loss of −40 dB in differential signal of 1.65 Gbps can be compensated by, e.g., an equalizer (EQ) or clock and data recovery (CDR) (e.g., MAX3815 is commercially available from Maxim Integrated Products, Inc.).
Use of equalizer or clock and data recovery for differential signals allows intersymbol interference (ISI) of differential signal to be compensated, thereby solving a problem of inter-pair skew in differential signals (a difference in signal propagation time between plural pairs of conductors). However, the problem of intra-pair skew (a difference in signal propagation time between a pair of conductors) has not been solved yet. Thus, the transmission range limit and transmission rate limit of differential signal depend on intra-pair skew of differential transmission cable.
In addition, along with a sharp increase in communication traffic and improvement in semiconductor technology allowing its implementation, differential transmission cables are required to have a further increased transmission rate. For this, it is desired to reduce intra-pair skew of differential transmission cable.
As a conventional differential transmission cable with reduced intra-pair skew, a low-skew parallel coaxial cable has been proposed in which one or more pairs of inner conductors extending in parallel are covered all together with a foamed insulation having a circular or oval cross section, an outer conductor is provided around the foamed insulation, and the outer conductor together with the foamed insulation is covered with an insulating jacket without any gaps (see, e.g., PTL 1).
Meanwhile, a differential signal transmission cable has been proposed in which a pair of conductors arranged in parallel is covered with a flat insulation having flat portions which face each other in a direction orthogonal to an arrangement direction of the pair of conductors so that the pair of conductors is sandwiched therebetween, a shield conductor formed of a metal foil tape is wound around the outer periphery of the insulation, a drain wire is placed longitudinally so as to be in contact with the shield conductor at the position of the flat portions, and the drain wire and the shield conductor are covered with a jacket. In this configuration, electromagnetic coupling between the conductors is increased by reducing a distance between the conductors and common-mode impedance is thereby increased (see, e.g., PTL 2).